docs: [FC-0074] replace warning text with event_warning annotation

[mariajgrimaldi]

Description

Use .. event_warning annotation to indicate a critical characteristic of an event. As for now, these warnings have been used to warn users about missing event bus capabilities.

Testing Instructions

1. Create a venv
2. Install this version of openedx-events which uses .. event_warning instead of a Warning: label.
3. Generate docs for openedx-events by running: make docs

Here's the expected output:

This PR depends on openedx/code-annotations#136

[openedx-webhooks]

Thanks for the pull request, @mariajgrimaldi!

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[sarina]

Reviewed at https://docsopenedxorg--431.org.readthedocs.build/projects/openedx-events/en/431/reference/events.html